Field of the Invention
The invention relates to a method and a device for operating an asynchronous machine.
Recent studies in Japan, the USA and Europe have shown that variable-speed machine sets have substantial advantages in part over fixed-speed systems. That holds true, in particular, for machine sets in hydroelectric installations with sharply fluctuating heads, in which the efficiency can be optimized by speed adjustment both in the generator mode and in the pump mode.
In hydroelectric installations having variable-speed machine sets, the machines are connected to the mains through static frequency converters. As a result the mains frequency and machine frequency are decoupled from one another.
An article entitled "Static Frequency Converters for Adjustable-Speed Sets" by O. Warneke, in International Water Power & Dam Construction, June 1995, pages 36 to 38 discloses that a double-fed variable-speed asynchronous machine with a direct converter unit is one of the converter-machine variants most frequently employed in the range of medium and high power, that is to say from about 6 MW to several hundred MW. That system can be operated both in the motor mode and in the generator mode, with adjustable speed in each case. It is also necessary in that case to start up to motor mode from a standstill. It is possible, moreover, to brake to a standstill both from motor mode and from generator mode.
An explanation in a book entitled "Die Induktionsmaschinen" ("Induction machines") by R. Richter, Verlag Birkha user, Basel/Stuttgart (1954), Volume IV, discloses, inter alia, methods for operating machines in which the machines are optimized for doubly synchronous continuous operation.
In a first method, the stator winding and the rotor winding of the variable-speed asynchronous machine are connected in series and fed by the direct converter unit. That requires the use of additional circuit-breakers accompanied by a substantial outlay for costs.
In a second method, the stator winding and the rotor winding of the variable-speed asynchronous machine are connected in parallel. Since the ratio of the stator current to the rotor current is determined by the impedances thereof, and the asynchronous machine is optimized for mains operation, that is to say no optimization is provided with respect to start-up and braking, the result is only an inadequate torque.
Similar methods are likewise disclosed in Published European Patent Applications 0 200 081 A2 and A3 and an article entitled "A High Performance Parameter-Insensitive Drive Using a Series-Connected Wound Rotor Induction Motor" by Edward Ho and Paresh Sen, in IEEE Transactions on Industry Applications, 1989, Vo. 25, No. 6, pages 1132 to 1138.
Furthermore, German Published, Non-Prosecuted Patent Application 2 150 531, U.S. Pat. No. 4,132,931 and an article entitled "The Double-Fed Induction Motor", by D. Lecocq, D. Lataire and W. Wymeersch, in EPE Journal, Vol. 1, Issue 2, 1991, pages 103 to 112, disclose asynchronous slip ring machines which respectively have a direct converter in the stator circuit and the rotor circuit for start-up and braking.
When optimizing the machine for continuous operation with a direct converter unit, the known methods prove to have problems if the aim is to start up and brake in accordance with a doubly synchronous method.
It is normally starting motors or starting turbines, circuits for reduced-voltage starting or special starting converters with indirect converters which are used to start up the variable-speed asynchronous machine. That results in a large outlay for apparatus and funds.